The present invention relates to an actively cooled device including bodies of heat resistant (i.e. refractory) material which are each surface brazed to at least one coolant conduit.
Many purposes require highly thermally stressable shields, the so-called "heat shields". Typical examples are the diverters and limiters of a fusion reactor. Since heat shields are a preferred field of application for the invention, it will hereinafter be described for the example of heat shields. However, the invention is not limited to them; it can also be used for other actively cooled devices, such as cooled drawing dies for the production of profiled rods, cooled electrodes for fusion electrolysis and the like.
If it meets certain quality requirements, graphite, due to its characteristics, is a good heat shield material for plasma physical systems, such as fusion reactors, and other vacuum systems. Low atomic weight, high sublimation temperature, good heat conduction, and low atomization rate are some of these characteristics. On the other hand, graphite also has various drawbacks, such as porosity, low mechanical strength and low ductility.
The porosity of graphite generally forbids, already for density reasons, direct contact of the graphite with a cooling fluid. Its poor mechanical strength and ductility make the joining of graphite parts more difficult and limit the maximum temperature under which it can be used. In the past, therefore, graphite elements serving as heat shields could be cooled only by radiation or by thermal contact with a heat dissipating fastening structure. The graphite elements of such a known heat shield in an experimental fusion reactor, such as a Tokamak, are therefore adiabatically heated during a plasma discharge which lasts up to about 10 seconds and then require at least about 10 minutes to cool sufficiently. Longer plasma discharges or stationary operation is therefore impossible if the prior art graphite heat shields are used.
Similar problems exist with the so-called first wall of fusion reactors. In this connection, the JAERI-M82-174 (1982) report discloses the soldering of silicon carbide plates to a planar frontal face of strip-like projections formed in a base plate by cooling channels having a trapezoidal cross section. The Journal of Nuclear Materials 103 & 104 (1981) pages 31-40, further discloses a limiter which contains water cooled copper plates to whose surface graphite tiles are brazed. However, the strength of these planar brazed connections leaves something to be desired under the unavoidable alternating temperature and pressure stresses, and cracks frequently develop at the solder locations which greatly impede heat transfer.
Consequently, the present invention is based on the problem of assuring, in a device, e.g. a heat shield, employing elements made of a heat resistant material which are actively cooled by a coolant, the durability of the brazed connections and thus reliable dissipation of heat from the heat resistant elements to the coolant so that greater stressability and/or longer service life result.